Polyolefin multilayer films, especially polypropylene based films, are widely used in packaging applications, such as pouches for dry food mixes, pet foods, snack foods, and seeds. In many film applications it is desirable to seal the film during the packaging operation. This may be accomplished by the use of adhesives or by using heat sealing techniques. When heat sealing is used, it is important that the plastic film be readily heat sealable while also possessing other good physical and mechanical properties such as resistance to tearing, high tensile strength, and good processability in high speed equipment. Such multilayer films preferably have the ability to form strong seals at relatively low temperatures and, in some instances, have the ability to do so in the presence of contamination in the seal region from the contents of the pouches.
U.S. patent application Ser. No. 11/096,298 discloses multilayer film wherein a soft polymer is blended in a core layer and a tie layer comprising the soft layer and, optionally, another polymer. A sealable layer is provided on the side of the tie layer opposite the core layer. The multilayer films may be transparent, contain a cavitating agent, or are pigmented to form an opaque film. Also, the multilayer film may be metallized or coated with a barrier coating.
U.S. patent application Ser. No. 11/248,838 discloses multilayer films including a core layer, a tie layer made from at least 10 wt. % of a first polymer and where the first polymer preferably is not present in the core layer. Optionally, the multilayer film may have a skin layer, a second tie layer, and/or a second skin layer.
U.S. patent application Ser. No. 11/521,657 discloses multilayer films including a core layer, a tie layer made from at least 10 wt. % of a first polymer and a service layer, wherein the tie layer is a sealable layer and may provide a hermetic seal when sealed to itself. Optionally, the multilayer film may have a skin layer and/or a second skin layer.
PCT Application No. WO 2007/047133 discloses heat-sealable, multilayer composite packaging structures. The film structure includes a first substrate, such as paper, bonded such as by extrusion lamination, to a sealable, high-barrier film including in this order: (1) a core layer comprising from about 5 wt. % to about 40 wt. % of a first polymer, wherein the first polymer includes a density in the range of 0.850 g/cm3 to 0.920 g/cm3, a DSC melting point in the range of 40° C. to 160° C., and a melt flow rate in the range of 2 dg/min to 100 dg/min; (2) a tie layer comprising said first polymer; and (3) a sealing layer, the tie layer being on a side of the core layer opposite the first substrate.
U.S. patent application Ser. No. 11/588,204 discloses heat sealable films having a heat sealable layer comprising a blend of propylene-based polymers.
U.S. patent application Ser. No. 11/804,630 discloses polypropylene films with moisture barrier properties. The multilayer films include a core layer having at least one nucleating agent and at least one water vapor transmission inhibitor. Optionally, the multilayer film may have at least one skin layer and at least one tie layer located intermediate the core layer and the at least one skin layer.
U.S. Pat. No. 6,844,078 discloses a coextruded multilayer film, having of at least a high crystalline propylene homopolymer resin layer of isotactic content greater than about 95%; a discharge-treated surface on one side of said polyolefin resin layer, an amount of hydrocarbon resin up to 10 wt. % of the high crystalline propylene homopolymer of greater than about 95% isotactic content; and on the high crystalline propylene homopolymer resin layer side opposite said discharge-treated surface, a heat sealable or winding layer having an antiblock.
PCT Application No. WO 2010/033276 discloses multilayer films which include at least one core layer and a sealing layer, and optionally a first tie layer intermediate the core layer and the skin layer. The sealing skin layer comprises a first polymer component having a heat of fusion of less than 75 J/g and a second polymer component. The multilayer film preferably has a seal strength greater than about 200 g/2.54 cm for a seal formed on a crimp sealer at a temperature of at least 93.3° C. The core layer may comprise a nucleating agent and a hydrocarbon resin. The multilayer film may be biaxially oriented and may be useful in packaging applications. These films contain less than 50% second polymer component in the sealing layer.
Opportunities exist for polymer films, particularly biaxially oriented polypropylene films, to replace other packaging substrates, such as paper and foil, in many temperature-sensitive packaging operations, such as with ice cream bars, chocolate bars, and dry-particulate foods. There remains a need for films having improved mechanical and optical properties (such as seal initiation temperature, hermeticity, hot tack strength, and seal temperature ranges) that also demonstrate good processability and orientation stability. In particular, there is a need for films that can be used in high-speed packaging equipment.